In a TDD (time division duplex) system, a base station performs estimation on an uplink channel by detecting an SRS (sounding reference signal) signal sent by a UE (user equipment) and uses an estimated uplink channel as a downlink channel to perform beamforming according to reciprocity of an uplink and a downlink of the TDD system. Theoretically, an uplink and a downlink in the TDD system are reciprocal; however, in an actual system, different intermediate radio frequency transceiving channel responses in the base station are respectively introduced to uplink and downlink channel responses. By means of theoretical analysis, it is found that if ratios of transmit channel responses to receive channel responses of any two intermediate radio frequency channels are different, an effect of coherent transmission is affected. Therefore, compensation needs to be performed on each intermediate radio frequency transceiving channel, which is referred to as channel calibration. Currently, there is a mature method for channel calibration for a single cell.
CoMP (Coordinated Multi-Point, coordinated multi-point transmission) is a key feature of an LTE (Long Term Evolution) system. A problem of interference on an edge of an LTE cell may be effectively resolved by using a CoMP technology. A base station side in the LTE system may include a BBU (baseband unit), and each BBU may be connected to multiple RRUs (remote radio unit). As one type of CoMP technology, a JT (joint transmission) technology may significantly increase cell edge throughput and average throughput. To implement a gain brought by the JT technology in the TDD system, channel calibration needs to be performed as well in the TDD system. Different from a single-cell beamforming technology, the JT technology requires that ratios of uplink radio frequency channel responses to downlink radio frequency channel responses are the same, where the uplink and downlink radio frequency channel responses are corresponding to various antennas of multiple cells. Even though channel calibration is performed on each cell, if calibration among cells is not performed, the JT technology still cannot achieve expected performance. Currently, a channel calibration technology for a single RRU is relatively mature; however, joint calibration among multiple RRU channels is still a difficult problem in the industry.